A known device of the state of the art successively comprises:                a substrate comprising a metal layer capable of reflecting the radiation and of conducting an electric current;        a first layer of a III/N-type alloy, p-type doped, and comprising a first surface, opposite the metal layer, provided with cavities;        a light-emitting layer of a III/N-type alloy, capable of generating the radiation;        a second layer of a III/N-type alloy, n-type doped, having the radiation coming out therethrough. The second layer has an external surface forming an interface with the exit medium. The outer surface of the second layer is textured to avoid for a major part of the generated radiation to be trapped within the device by internal total reflections.        
Conventionally, such a device of the state of the art is obtained by successive epitaxies of the second layer, of the light-emitting layer, and of the first layer on a growth substrate, made of sapphire or of AlGaN. Then, the metal layer is formed on the epitaxial layers. The assembly is then transferred onto a host substrate which is a good heat conductor. Finally, the growth substrate is suppressed by laser lift-off. The texturing of the outer surface of the second layer is obtained by a selective chemical etching based on KOH.
Crystal mesh and thermal expansion coefficient mismatches between sapphire and III/N-type alloys result in dislocations through the epitaxial layers. The dislocations result in the presence of defects in the epitaxial layers. More specifically, III/N-type alloys have a wurtzite-type hexagonal crystal structure, and the defects take the shape of cavities, emerging at the level of the dislocations, particularly at the first surface of the first layer. The cavities conventionally form hollow patterns in the shape of an upside-down pyramid having a hexagonal base (V-pits); the tops of the pyramids point towards a dislocation or a group of dislocations. Such cavities form a natural texturing of the first surface of the first layer, and are filled with metal on forming of the metal layer.
Such cavities would be advantageous for the extraction of light, due to their diffusing power, as suggested in C. M. Tsai et al., “High efficiency and improved ESD characteristics of GaN-based LEDs with naturally textured surface grown by MOCVD”, Photonics Technology Letters, IEEE, vol. 18 (11), 2006, pp. 1213-1215.
However, as illustrated in FIG. 5, the applicant has observed that the total reflection, that is, the specular and diffuse reflection, of the radiation at the interface between the metal layer and the first surface of the first layer decreases when the cavity density increases, which is prejudicial to obtain a high light extraction efficiency (that is, greater than 50%) and a high efficiency of the device, the device efficiency being the ratio of the available optical watts to the injected electrical watts.